The present invention relates generally to force measurement instrumentation, and more particularly to instrumentation adaptable to the special challenges of micromechanical applications.
The development of practical micromechanical devices which can be operated reliably and manufactured routinely and with high process yield is currently hampered by a virtual absence of standard diagnostic instrumentation. Such fundamental parameters as physical structure, displacement distance, spring constants, fracture strength, forces, and many others cannot at present be measured routinely. Relative values for such parameters can at times be inferred from operating voltage, measured capacitance, and the like, but such indirect estimates fail badly when absolute accuracy is needed.
Among the practical problems generated by this lack are the difficulty of efficiently designing a set of devices which are intended to function properly and reliably with each other. Owing to process variations and independent development, a device which in its ideal design will take 8 xcexcNewtons to operate and an actuator which ideally will deliver 12 xcexcNewtons may not function properly together when integrated into and fabricated on a single chip. Modification of the fabrication process also can result in unexpected changes in functionality for complex devices. The ability to measure absolute fundamental material and device properties would greatly reduce the time and expense required to make such adjustments.
Several approaches exist to estimate relative forces in specific situations. For example, for a parallel plate electrostatic actuator, the output force can be estimated given the area of the plates, the voltage difference between the plates, and the distance between the plates. In most cases, this works fairly well for large plates and near-zero displacements. However, because no technique exists to directly calibrate the force produced by such an actuator, many potentially defective assumptions must be made to calculate the actuator force. Among these are that the areas for the plates are correct, that the plates are flat, that the plates are parallel, that the plate supports have not warped from residual stress (which could change the plate""s separation at rest), that doping or interlayer electrical difficulties do not reduce the voltage or charge being applied to the plates, and so on. With most of these problems, the actuator will give 4 times the force when twice the voltage is applied, so relative measurements can be made using this approach, but absolute measurements do not presently appear practical.
Another approach which has been used is to measure the bending of a cantilever under an applied force. The equations describing the bending of a cantilever are then used to extract the force being applied. This approach has a number of problems. As before, assumptions concerning the dimensions and structure of the cantilever and its anchor have to be made to do the analysis.
Beyond that, the strain acting on a bent cantilever is highly inhomogeneous owing to the stress concentration associated with the cantilever anchor. The response of the cantilever therefore depends sensitively on structural flaws, particularly those which may exist near the anchoring structure.
Also, the strains encountered in making practical force measurements in the realm of micromechanical devices are a sufficiently large that the material from which the cantilever and at least part of its support are made exhibit nonlinear mechanical responses. This factor complicates the analysis, and makes the measurement even more susceptible to process variations and similar unintended factors.
Accordingly, there is a long-felt need for a micromechanical device which can measure absolute forces with reasonable accuracy. Ideally, such a device would be integrable with production microelectromechanical systems (MEMS), and could be calibrated independent of other MEMS devices. Further, interpreting the output of such a device would be simplified if the basic design required limited material strain for operation. Finally, real-time diagnostics for proper functioning of the device would be useful.
A new class of micromechanical dynamometers is disclosed. The combination of the small size scale and the enormous strength of micromechanical materials allows a wide range of applied forces to be measured directly. These dynamometers can be externally calibrated against a reference.